Diesel fuels and/or biodiesel fuels typically contain wax and when subjected to low temperatures these fuels often undergo wax crystallization, gelling, and/or viscosity increase. This reduces the ability of the fuel to flow and creates filter plugging which adversely affects the operability of vehicles using these fuels. Flow improvers have been used to modify the wax structure as it builds during cooling. These additives are typically used to keep the wax crystals small so that they can pass through fuel filters. Also, pour point dispersants are sometimes used in diesel fuel to ensure that it can be pumped at low temperatures.
Due to environmental concerns and the decline of known petroleum reserves with subsequent price increase of petroleum, biodiesel fuels are becoming a focus of intense research and development efforts. Biodiesel fuels typically comprise fatty acid esters, prepared for example by transesterifying triglycerides with lower alcohols, e.g. methanol or ethanol. A typical biodiesel fuel is the fatty acid ester of a natural oil, with non-limiting examples of natural oils such as canola oil, rapeseed oil, coconut oil, corn oil, cottonseed oil, olive oil, palm oil, peanut oil, safflower seed oil, sesame seed oil, soybean oil, sunflower oil, linseed oil, palm kernel oil, tung oil, jojoba oil, jatropha oil, mustard oil, camellina oil, pennycress oil, hemp oil, algal oil, castor oil, lard, tallow, poultry fat, yellow grease, fish oil, tall oils, and mixtures thereof. Optionally, the natural oil may be partially and/or fully hydrogenated, and may also be refined, bleached, and/or deodorized. One of the major problems associated with the use of biodiesel is its poor cold flow properties resulting from crystallization of saturated fatty compounds in cold conditions, as indicated by its relatively high cloud points (CP) and pour points (PP). A 20° C. reduction in cold filter plugging point is necessary for some biodiesel fuels to find utility in colder climates such as those of North America and Europe in winter.
Several efforts to mitigate the low-temperature problems of biodiesel have been investigated over the past several years. Many popular approaches have included blending biodiesel with conventional diesel fuel, winterization, and use of synthetic additives. Also, studies have been performed to show the diversification in the feedstock and genetic modification of the feedstocks aimed to provide a reduction in the saturated content of the fatty acid methyl esters (FAME) in biodiesel as well as modification of FAME composition/profile of the fuels. While there have been efforts to create additives that may reduce the PP and cold filter plugging point (CFPP) of fuels, many are not cost effective. Also, increasing the unsaturated content of biodiesel may improve its cold flow properties, but leads to the alteration of the oxidative stability of the fuel. The overall thermal behavior of biodiesel is affected by the relative concentration of its saturated and unsaturated FAME components. The cold flow issue is primarily a multifaceted problem of crystallization (of saturated FAMEs) in solution (unsaturated FAMEs) which can be approached from several angles. Studies of the phase behavior of the individual FAMEs and mixtures constituting the biodiesel have already been used as a means to better understand the thermodynamics and kinetics of phase change in biodiesel. Phase diagrams of FAME systems are particularly investigated and modeled to provide an understanding of the molecular interactions involved, intersolubility and detection of special transformation points such as eutectics, peritectics and compound formation.
We have found that studying the phase behavior of the individual components of biodiesel, as well as their combined mixtures, helps understand the fundamental mechanisms of their crystallization so as to design biodiesel with improved low temperature characteristics. Fundamentally, the objective would be to adequately disrupt the crystallization process at both the nucleation and growth stages in order to lower the onset temperature of crystallization and decrease the number and size of the crystals. In this regard, a better understanding of the phase behavior of the biodiesel components and any potential additive which is an “improver” of cold flow or any other property is of key importance.
The development of specific thermodynamic models for predicting crystallization/melting behavior of biodiesel and biodiesel/additive would be a valuable tool in industry and commercial applications. In particular, we have studied binary phase behaviors of certain triacylglycerols (TAGs) such as 1,3-dioleoyl-2-palmitoyl glycerol (OPO) and 2-stearoyl diolein (OSO), and fatty acid methyl esters (FAMEs) such as methyl palmitate (MeP) and methyl stearate (MeS), and/or mixtures thereof.